The present invention relates to a method and to a mass spectrometer and uses thereof for detecting ions or subsequently-ionised neutral particles from samples.
Methods and mass spectrometers of this type are required in particular for determining the chemical composition of solid, liquid and/or gaseous samples.
Mass spectrometers have a wide application in determining the chemical composition of solid, liquid and gaseous samples. Both chemical elements and compounds and also mixtures of elements and compounds can be detected via determination of the mass-to-charge ratio (m/q), subsequently termed “mass” for simplification. A mass spectrometer consists of an ion source, a mass analyzer and an ion detector. There are various types of mass analysers, amongst those inter alia are time-of-flight mass spectrometers, quadruple mass spectrometers, magnetic sector field mass spectrometers, ion trap mass spectrometers and also combinations of these types of equipment. The ion production is effected according to the type of sample to be analyzed via a large number of methods which cannot be listed here completely. Thus there is used for the ionisation in the gas phase, e.g. electron-impact ionization (EI), chemical ionisation (CI) or ionisation by a plasma (ICP); for liquids, there are used inter alia electrospray ionisation (ESI), for solids inter alia, desorption methods, such as laser desorption (LD, MALDI), desorption by atomic primary ions or cluster ions (SIMS) and field desorption (FD). Desorbed neutral particles can be subsequently ionised by electrons, photons or by a plasma and thereafter analyzed by a mass spectrometer (SNMS).
FIG. 1 shows a time-of-flight mass spectrometer of this type having an ion source 1, a time-of-flight analyzer 2, a detector/signal amplifier 3 and an electronic recording unit 4. The time-of-flight analyzer 2 is passed through by an ion beam 11 in which ions m1, m2 and m3 of different masses pass through at intervals.
In this time-of-flight mass spectrometer, the ions 11′ 11″, 11′″ are extracted from the ion source 1 and then generally accelerated to the same energy. Subsequently the flight time of the ions in the time-of-flight analyzer 2 is measured with a defined flight distance. The starting time is established by a suitable pulsing of the ion source itself or by a pulsed input into the time-of-flight analyzer 2. The arrival time of the ions is measured by a fast ion detector with signal amplification 3 and a fast electronic recording unit 4.
The flight time in the time-of-flight spectrometer is proportional to the root of the mass in the case of the same ion energy. By means of suitable ion-optical elements, such as ion mirrors (reflectron) or electrostatic sector fields, different starting energies or starting positions of the ions with respect to the time-of-flight can be compensated for so that the time-of-flight measurement enables a high mass resolution (separation of ions with a very low mass difference) and high mass precision. The essential advantages of the time-of-flight spectrometer relative to other mass spectrometers reside in the parallel detection of all masses which are extracted from the ion sources and an extremely high mass range. The highest still detectable mass is produced from the maximum flight time which the electronic recording unit detects.
The relative intensity of the different masses in a single measurement can be determined from the level of the pulse response of the fast ion detector. However, generally it is not the result of a single flight time measurement which is evaluated but rather the events are integrated over a large number of cycles in order to increase the dynamics and the accuracy of the intensity determination. According to the dimensioning of the time-of-flight spectrometer and the highest mass to be recorded, the maximum frequency of these cycles is a few kHz to a few 10 kHz. Thus, for example at an ion energy of 2 keV, a typical flight distance of 2 m and a frequency of 10 kHz, a maximum mass of approx. 960 u is produced. Doubling the frequency reduces the mass range by the factor 4 to approx. 240 u.
A high mass resolution M/ΔM of 10,000 requires not only a suitable geometry of the analyzer for energy- and space focusing. It can only be achieved if the ion detector and the electronic recording unit enable a very high time resolution in the range of 1-5 as (M/ΔM=0.5×t/Δt). In particular with very low masses M with a relatively short time-of-flight t, the time resolution Δt should be better than 1 ns.
The ion detector should, for a high sensitivity, enable detection of single ions. For this purpose, the ions are converted into electrons by ion-induced electron emission on a suitable detector surface, and the electron signal is amplified by means of fast electron multipliers by typically 6-7 orders of magnitude. For potential separation, also arrangements are used in part, which convert the electrons by means of a fast scintillator into photons and then subsequently amplify the photon signal by means of a fast photomultiplier. The produced pulses are then evaluated with a fast electronic recording unit and the arrival times of the ions are determined with a precision of 1 ns up to a few 100 ps. For this purposes, the amplification in the ion detector must be effected such that the output pulses have as short a pulse duration as possible and such that flight time variations in the amplification process are minimized. In time-of-flight mass spectrometry, micro channel plates (MCP) are therefore used very frequently and are distinguished by a planar detector surface and a particularly fast pulse response with pulse widths in the range of 1 ns. Since the amplification of a single MCP generally does not suffice, arrangements of typically 2 MCPs in succession or of one MCP with scintillator and photomultiplier are used in order to achieve a total amplification of 106 to 107. In addition, also other types of electron multipliers, e.g. with discrete dynodes, are in use.
The dynamic range is of great importance for the use of mass spectrometers. The ratio of the highest signal to the smallest signal which can be recorded is herewith described. In the case of too high signals, the intensity is not measured correctly (saturation limit) as a result of saturation effects of the detector or of the recording. In the case of too low signals, the signal cannot be separated from noise or from the background. The dynamic range of a time-of-flight spectrometer is determined essentially by the detector and by the recording method. If the dynamic range is very small, then the intensity extracted from the pulsed ion source must be adapted very precisely to the dynamic range. The maximum intensity should still be below the saturation limit. The dynamic range then directly determines the detection limit of the time-of-flight mass spectrometer. Within the dynamic range, the measurement of the intensities should be as precise as possible in order that relative intensities, such as isotopic distributions and relative concentrations, can be determined correctly.
A type of recording which is used very frequently in time-of-flight mass spectrometers is based on a single particle counting technique with time-to-digital converters (TDC). The detector delivers for each detected ion an output pulse above a discriminator threshold and the precise arrival time is determined from the pulse response of the detector, e.g. according to the constant-fraction principle. With this technique, the time-of-flight can be measured with a very high time resolution of approx. 100 ps. Immediately after detection of an ion, a dead time of a few as to a few 10 ns results. Within this dead time, no further ions can be detected. This type of recording is therefore suitable only for relatively low counting rates. By means of accumulation of the single article events over a large number of cycles, a histogram of the arrival times can be produced, which provides the intensities of the different masses with sufficient dynamics. In the case of a frequency of 10 kHz, approx. 105 ions in the most intensive mass line (peak) can be recorded thus in 100 s (106 cycles). In the case of a frequency of 10% for detection of an ion in the highest peak, the probability of a second ion arriving within the dead time of the recording is still relatively low in the range of a few %. At higher counting rates, the probability of multiple ion events increases however significantly. Since the recording records respectively only one single event even in the case of multiple ion events, too few ions are counted in the relevant peak (saturation). This leads to significantly falsified relative peak in densities. These saturation effects due to the occurrence of multiple ion events can be reduced by application of a statistical correction, subsequently termed Poisson correction (T. Stephen, J. Zehnpfenning and A. Benninghoven, J. Vac. Sci. Technol. A 1994, 12, p. 405). Sufficient measuring accuracy for the most intensive peak can be achieved by the Poisson correction up to a frequency of approx. 80%. This corresponds approximately to an average number of entering ions of approx. 1.6. The statistical measuring error is then approx. 0.12% in the case of 106 cycles.
Higher counting rates than approx. one ion per mass and cycle can generally not be measured with sufficient accuracy in the single particle counting technique, even when using the Poisson correction. This saturation limit determines the maximum possible dynamic range of time-of-flight mass spectrometers for a specific frequency and measuring time. The dynamics in this type of operation can only be improved by increasing the number of cycles with a corresponding accompanying extension of the measuring time.
The counting rates can be increased if a plurality of ions per cycle and mass line can be recorded at the same time. A series of techniques has been developed here, which can be explained subsequently only in part. A description of some techniques is found for example in U.S. Pat. No. 7,265,346 B2.
A plurality of independent detectors in the single particle counting technique with TDC recording can thus be connected in parallel. In the case of homogeneous illumination of all detectors, each detector can detect at most one ion per cycle. The technical complexity hence increases significantly with the number of detectors so that typically only a small number of detectors is used in parallel. The dynamic range is hence typically increased by less than a factor of 10. The different detectors can be equipped both with the same and with a different detector surface.
As an alternative to using a plurality of parallel detectors, recordings can also be used which measure the pulse amplitude of the ion detector and determine the number of simultaneously arriving ions from the pulse amplitude. For this purpose, fast analogue-to-digital converters (ADC) which have a high sampling rate and bandwidths in the GHz range are used. Typically, the dynamics at the respective bandwidth up to some GHz are approx. 8-10 bit. The pulse response of a typical ion detector with MCP for a single ion has generally however a relatively wide pulse height distribution. Since a sufficiently high proportion of the single particle pulses must be still significantly above the noise level of the ADC (lowest bit) in order to ensure a high detection probability, a significant fraction of the dynamic range of the ADC is already used even for a relatively low number of ions. The detector amplification must be chosen very carefully in order to avoid saturation of the ADC and at the same time to keep the discrimination of low peak intensities (single ions) low. In order to suppress the noise of the ADC (lowest bit), a suitable threshold is defined and the signals below this threshold are not taken into account during the integration of the data over a large number of shots. This suppression of a part of the single ions leads to non-linearity of the recording in the transition range from the single ion detection to multiple ion detection. In fact, in the case of careful calibration of detector and recording, corresponding corrections of the intensities can be implemented. However, high accuracy of the intensity measurement can be achieved only with great difficulty with such an arrangement. The measurement of large intensity ratios with an accuracy of better than 1% is hence impossible.
The dynamic range can be increased by the parallel use of two ADCs with a different amplitude measuring range. In the case of saturation of the ADC which records the single ions and the low intensities, the high signals are detected with a second ADC. Both measuring results must then be combined suitably to form one spectrum. The dynamics can then be increased up to approx. 12 bit. In this way, up to a few hundred ions per cycle on one mass can be detected. Since these high intensities can however result in saturation effects in the MCPs, the accuracy of the intensity measurement when using fast MCP detectors is not very high. The output current of the MCPs, in the case of sufficiently high amplification, is no longer sufficiently proportional to the input current. Furthermore, the lifespan of the MCP detector is significantly reduced in the case of these high counting rates and the amplification reduces with the number of detected ions. A further disadvantage of the ADC solution resides in the reduced time resolution of detector and ADC in comparison with conventional TDC recording. Furthermore, an extremely high processing speed of the data is required when using ABCs in the GHz range and with shot frequencies of approx. 10 kHz. The technical complexity with these recording systems is therefore very high.
In the case of a large number of applications of time-of-flight mass spectrometry, intensities of different masses with very high dynamics and very high accuracy must be measured.
This applies for example to the measurement of isotopic ratios for elements with greatly differing isotopic abundances. Thus, for example the relative frequency of the isotopes of oxygen 16O/18O is approx. 487. If the single particle counting technique with TDC recording is used and if the signal is corrected by means of the Poisson correction, then at most approx. 1×106 ions of the type 16O can be recorded in 106 cycles. The intensity of the main isotope must be correspondingly optimized for this purpose. The simultaneously measured intensity of the isotope 18O is then only approx. 2,055 ions. Hence, the statistical error for 18O is still at 2.2%. In order to reduce the statistical error to approx. 0.1%, the number of cycles must be increased by a factor 500 to 5×108. In the case of the typical frequencies of 10 kHz, a measuring time of approx. 14 hours is calculated for a statistical accuracy of 0.1%. Long measuring times of approx. 10 hours are likewise produced in the determination of other important isotopic ratios, such as e.g. of 238U/235U, 14N/15N, 12C/13C with high statistical accuracy.
A similar problem is shown also in the detection of traces in the ppm or ppb range. The intensities of the mass lines of the main components should still be below the saturation limit of the single particle counting technique (approx. 1 ion per cycle when using the Poisson correction), While, for the low concentrations, sufficient signal for a still adequate statistical accuracy must be accumulated. For a statistical accuracy of 1% with a detection limit of 1 ppm, approx. 1010 cycles are then required and hence measuring times of approx. 50 hours (a frequency of 20 kHz being assumed). The detection off 10 ppb with approx. 10% statistical accuracy requires approx. a comparable number of measuring cycles.
In other important types of operation, only very short measuring times are often available for the intensity determination. Thus, frequently temporally variable intensities with a time resolution in the range of a few seconds must be measured. Correspondingly, the number of measuring cycles for this time interval is still only approx. 105. The dynamics in the mass spectrum for this time interval is therefore reduced to approx. 4-5 orders of magnitude. The detection limit with a measuring time of 10 s is therefore, even in the case of optimum adaptation of the intensity of the main components, well above 1 ppm. A statistical accuracy of approx. 10% is given only above 1,000 ppm.
In the case of mass spectrometers for measuring distribution maps, generally the intensities must be measured for a large number of pixels. In the case of a relatively long measuring time of 1 hour, 256×256 pixels and a frequency of 20 kHz, only 1,100 measuring cycles per pixel are hence accumulated. The simultaneous measurement of distribution images for isotopes with a very different isotopic abundance, such as e.g. 16O/18O, is hence impossible in the single particle counting technique. The same applies for the measurement of distribution maps of masses with very different concentrations.